There is a law known as Section 508 which requires federal agencies to make their electronic data accessible to people with disabilities. While 508 compliance is of course not required for most websites, it is a good standard to adhere to, for a number of reasons. You always want visitors to your site to be able to find what they need without a problem. And if you want search engines to easily find your content, 508 compliance will improve your SEO as well.

508 compliance focuses on making the site more usable for a visitor viewing the site, such as giving the option to enlarge text or using colors that are easily distinguishable. But it also includes making content easily accessible and understandable to screen readers - for example, giving images "alt" (or tooltip) text which can be read aloud by screen readers.

Even if you don't expect many visitors with disabilities to your site, 508 compliance is still a worthwhile investment, because search engines look at and index content in a manner very similar to screen readers. So any content which a human visitor would not be able to find on your site, a search engine almost definitely will not be able to. For more information, visit the Section 508 website.